The present invention relates to a distance measuring device, in which sensitivity of a photodetection unit of a distance measuring device is easily adjustable, and the invention also relates to a method for adjusting the photodetection unit of the distance measuring device.
In distance measurement based on a light wave, a modulated measuring light beam is used, and a distance is determined from a phase difference of the reflected light beam. FIG. 4 is a drawing to explain a basic type of a distance measuring unit 1 of a distance measuring device, and FIG. 5 is a block diagram to explain the control unit 13 of the distance measuring device.
When a distance measuring light beam 4 is emitted from a light emitting element 3 which is modulated and driven by an oscillator 2, the distance measuring light 4 is turned to a parallel beam by an objective lens 5 and this beam is projected to a reflection prism 6, which is an object to be measured. The distance measuring light 4 reflected by the reflection prism 6 enters the objective lens 5 again and is received by a photodetection element 7. A part of the distance measuring light beam emitted from the light emitting element 3 is reflected by a reflection mirror 9 and it is received by the photodetection element 7 as an internal reference light beam 12.
Based on photodetection signals of the received distance measuring light beam 4 and the received internal reference light beam 12, the distance is calculated from the phase difference by a distance measuring circuit 8.
The distance to the object to be measured (the reflection prism 6) as obtained by the distance measuring circuit 8 is displayed on a display unit 15 as a numerical value indicating the distance based on the control by a CPU 14. Distance measuring instruction, data, etc. are inputted from a keyboard unit 16.
FIG. 6 is a drawing to explain the distance measuring unit 1 more concretely.
On the side of the light emitting element 3, a chopper 17 is provided to cut off the distance measuring light beam 4 coming from the light emitting element 3 such as LED, LD, etc., and the chopper 17 is rotated by an optical path changing motor 18. On the side of the photodetection element 7, a density filter 20 is provided to filter a reflected distance measuring light 4xe2x80x2, and the density filter 20 is rotated by a density adjusting motor 21. As the photodetection element 7, APD (avalanche photodiode) is used depending on the conditions such as photodetection sensitivity.
The chopper 17 is provided with such a pattern that the light beam on the inner side and the light beam on the outer side are alternately cut off. When the chopper 17 is rotated by the optical path changing motor 18, the optical path is changed alternately. The density filter 20 is provided with such a pattern that the density is gradually changed in rotating direction. When a rotating position of the density filter 20 is selected by the density adjusting motor 21, the received light amount of the photodetection element 7 can be adjusted. The light amount of the reflected distance measuring light beam 4xe2x80x2 entering the photodetection element 7 is increased or decreased according to the distance between the reflection prism 6 and the distance measuring device. By properly selecting the position of the density filter 20, the received light amount of the photodetection element 7 can be maintained on a constant level.
Based on the photodetection light amount (i.e. the received light amount) of the photodetection element 7, the density adjusting motor 21 is rotated so that the light amount of the reflected distance measuring light beam 4xe2x80x2 and the internal reference light beam 12 entering the photodetection element 7 is maintained at a constant level, and the position of the density filter 20 is controlled. In synchronization with a reference signal issued by the distance measuring circuit 8, the optical path changing motor 18 is rotated and controlled, and the reflected distance measuring light beam 4xe2x80x2 and the internal reference light beam 12 entering the photodetection element 7 are switched over.
The avalanche photodiode (APD) as described above has highly accurate photodetection sensitivity and has a wide dynamic range, and it is used as a photodetection element. However, the characteristics of APD widely vary according to each individual element, and adjustment is necessary when it is incorporated in a distance measuring device. In particular, the adjustment is indispensable in case of a distance measuring device with high accuracy.
By changing a bias voltage, APD has an ability to adjust a gain multiplication of an electric current by receiving the light beam. The characteristics of the bias voltage and the output current of APD are shown in FIG. 7. As shown in FIG. 7, APD has a wide range of the gain multiplication, and it can be 150 times or more. When the bias voltage exceeds a certain level (Va), an avalanche phenomenon occurs. Therefore, in the adjustment of APD, the bias voltage is set to a value lower than the bias voltage to induce the avalanche while maintaining the needed gain multiplication.
Description will be given now on a conventional adjustment mode of APD.
In the past, a special-purpose adjustment tool has been required for the adjustment.
As shown in FIG. 8, a conventional type adjustment tool comprises a combination of an LED 22 for adjustment and a density filter 23.
First, a light beam is emitted from the LED 22, of which output light amount is determined at a constant level, and it is projected toward the photodetection element 7 at a predetermined position. The emitted light beam is modulated in the same manner as the actual distance measuring light beam 4.
APD directly receives the light beam from the LED 22, and the adjustment is made from a value obtained by photoelectric conversion. The gain multiplication in this case is assumed as 1xc3x97.
Next, a {fraction (1/150)} density filter 23 is inserted between the photodetection element 7 and the LED 22. The photodetection light amount of the APD is turned to {fraction (1/150)} of the initial value. When the photodetection light amount is decreased to {fraction (1/150)}, the bias voltage is increased in such manner that the output current from APD is equalized to the value when the density filter 23 is not used. By the equalized bias voltage, the gain multiplication of 150 times is obtained.
However, even with the photodetection element 7 thus adjusted, it sometimes happens that the accurate output current as set to 150 times may not be obtained depending on the frequency of the use of the distance measuring device, the course over time, etc. In such case, the product must be adjusted again. For this reason, in the past, the distance measuring device has been re-adjusted periodically or after the use of a predetermined period of time. As described above, a special-purpose adjustment tool and a measuring instrument are required for the adjustment, and the user of the distance measuring device cannot perform adjustment as desired, and the re-adjustment has been usually requested to the manufacturer. This means that complicated procedures are required each time the distance measuring device has to be sent back to the manufacturer for re-adjustment. Also, the distance measuring device cannot be used during the re-adjustment, and this causes much inconvenience to the user.
It is an object of the present invention to provide a distance measuring device and a method for adjustment, by which it is possible to adjust the photodetection element (APD) in an easy and convenient manner, to eliminate complicated procedures for the re-adjustment, and to exclude the limitation on the use of the distance measuring device due to the re-adjustment.
To attain the above object, the present invention provides a distance measuring device for measuring a distance, comprising a light emitting unit for emitting a measuring light beam toward an object to be measured, and a photodetection unit for receiving a reflected light beam reflected from the object to be measured, wherein the distance measuring device comprises a gain control unit for changing photodetection sensitivity of the photodetection unit by a bias voltage and a light amount switching unit for variably changing a light amount of the light emitting element which emits said light beam toward the photodetection unit. Further, the present invention provides a distance measuring device as described above, wherein the light emitting unit comprises a light source for emitting the measuring light beam and a light emitting element for adjustment for emitting a specifying light beam for adjustment, and the light amount switching unit can variably change a light amount of the light emitting element for adjustment. Also, the present invention provides a method for adjusting a photodetection unit of a distance measuring device which has a light emitting unit for projecting a measuring light beam toward an object to be measured, and a photodetection element for receiving a reflected measuring light beam reflected from the object to be measured, comprising the step of projecting a light beam from the light emitting element with a plurality of values of specifying light amount toward the photodetection element, and the step of determining a range of photodetection sensitivity of the photodetection element in response to a change of the light amount. Further, the present invention provides a method for adjusting a photodetection unit of a distance measuring device as described above, wherein the light emitting unit comprises a light source for emitting a measuring light beam and a light emitting element for adjustment for emitting a specifying light beam for adjustment, wherein a light amount of the light emitting element for adjustment can be variably changed. Also, the present invention provides a method for adjusting a photodetection unit of a distance measuring device as described above, wherein the light emitting unit attenuates light at a predetermined gain multiplication to a specifying light amount, and a bias voltage is determined in such manner that sensitivity of a predetermined gain multiplication of the photodetection element can be obtained according to a light attenuating condition. Further, the present invention provides a method for adjusting a photodetection unit of a distance measuring device as described above, wherein the light emitting unit sequentially emits the light beam of a specifying light amount in a plurality of stages, a bias voltage is sequentially changed in response to the light amount emitted by the light emitting unit, and the bias voltage of the photodetection element is determined when the light emitting unit finishes a light emission in a predetermined plurality of stages. light emitting unit finishes a light emission in a predetermined plurality of stages.